The polymorphism of quinacridone pigments is well known. For example quinacridone, also referred to as 5,12-dihydroquino-[2,3-b]-acridine-7,14-dione of the formula is known to exist in three major polymorph modifications.
The alpha (U.S. Pat. No. 2,844,484) and gamma (U.S. Pat. No. 2,844,581, U.S. Pat. No. 2,969,366, U.S. Pat. No. 3,074,950 and U.S. Pat. No. 5,223,624) polymorphs are bluish or yellowish red pigments. The beta polymorph is described in U.S. Pat. No. 2,844,485 as a violet form and in U.S. Pat. No. 4,857,646 as a magenta form while the delta polymorph obtained by sublimation of quinacridone is described as a yellowish red quinacridone in U.S. Pat. No. 3,272,821. Since the alpha and delta forms are unstable, only the red gamma, and the violet and magenta beta quinacridone are of commercial interest.
Several routes are described for the preparation of quinacridone. Numerous publications describe the oxidation of 6,13-dihydroquinacridone to quinacridone. For example, U.S. Pat. No. 5,840,901 describes the oxidation of 6,13-dihydroquinacridone using hydrogen peroxide as the oxidant. This unique process provides an economic and environmentally friendly method for the preparation of quinacridone pigments.
Additionally, the process offers the preparation of quinacridones and its solid solutions in its desired crystal modification as for example the beta or the gamma quinacridone. For example, U.S. Pat. No. 6,013,127 describes a process for the preparation of a quinacridone/6,13-dihydroquinacridone solid solution in its gamma crystal form. Such products show outstanding pigment properties. Generally, a pigmentary form is obtained in the synthesis step without the need of an additional finishing step.
U.S. Pat. No. 6,225,472 and U.S. Pat. No. 6,406,533 describe novel 6,13-dihydroquinacridone derivatives, which can direct and control the growth and/or crystal phase of pigment particles.
U.S. Pat. No. 6,264,733 describes new pigment particle growth and/or crystal phase directors of the formula (MO3S)m-Q-(CH2—(X)—(Y)n)o (II), wherein Q represents a pigment moiety, M represents a metal cation, quaternary N cation or H, X is an aromatic group or an aliphatic heterocyclic group with at least one 5 atom or 6 atom ring or a heteroaromatic group with at least one 6 atom ring and which is not a phthalimido group, Y is a sulfonic acid or carboxylic acid or salt thereof; m and n independent from each other represent a number from zero to 2.5; and o is a number from 0.05 to 4.
Although beta quinacridone can be advantageously produced by these known methods the pigments prepared by such processes are opaque or semi opaque and still need an after treatment or finishing like solvent treatment, further grinding or kneading or even precipitation from mineral acids like concentrated sulfuric acid to get a very small particle size C.I. Pigment Violet 19 in its desired color characteristics, high color strength and high transparency.
Furthermore, depending on the reaction conditions the beta quinacridone is often obtained in mixture with a small quantity of the red gamma or alpha quinacridone present, leading to a hue shift and lower chroma.
The present process describes a new route for the preparation of a pure single phase direct pigmentary beta quinacridone pigment by the oxidation of 6,13-dihydroquinacridone, in that the process is carried out in the presence of a specific polymer or a specified mixture of polymers and optionally in the presence of a small particulate quinacridone as a seed.
Thus, the inventive process is valuable since it offers a new viable environmentally friendly and economical route for the preparation of new violet quinacridone pigments.